My present invention relates to a method of and an apparatus for determining the speed of sound in a material as a function of its temperature.
For many applications a steel pipe or tubing is required which is seamless and is fabricated by passing a cylindrical solid workpiece of steel between a pair of mutually inclined rolls which cooperate with a fixed mandrel to form a passage in the workpiece. The production of seamless tubing by rolling the material over a mandrel is described, for example, in EP 0 940 193 A2. This method is known variously as the piercing process and the roll-forging process. In a stretch-reducing rolling and through the use of reducing rolling and dimensional rolling, a seamless steel pipe or tubing can be fed through a multiplicity of roll stands and the desired dimensions can be imparted to the workpiece so that a particular cross section is obtained. In each roll frame there may be three rolls which enable the pipe or tubing to be engaged on all sides. The rolling will generally reduce the diameter and impart a precise shape to the product.
The pipe or tubing after rolling should have an ideal shape in that the cylindrical contours of the inner and outer peripheries should correspond to two precisely concentric circles. In practice, however, there are always fabrication tolerances so that there is always some eccentricity of the circular contour of the interior of the workpiece relative to the circular contour of the exterior thereof.
This effect can be measured by detecting the wall thickness of the tubing and the process can be controlled in response to monitoring the wall thickness.
To detect the wall thickness and derive therefrom signals which can be used to control the process or to signal the wall thickness and thus the ability of the workpiece to meet tolerance requirements, especially for the hot workpieces, earlier methods have utilized laser and ultrasonic measurement techniques. Ultrasonic thickness measurements utilize a pulse echo method which, from a transit time of an ultrasonic pulse, can calculate the wall thickness. For this, however, it is necessary to know the speed of sound in the material of the workpiece at the temperature of the workpiece at which the measurement is to be made. The speed of sound in the material is thus dependent both upon workpiece composition and on the temperature.
From Canadian patent 2 187 957 A1, it is known to use ultrasonic pulses and monitoring for controlling process conditions in liquid metals. The principle involved is also a pulse echo method which evaluates the reflected ultrasound pulses.
For determining the speed of sound in materials at predetermined temperatures, a variety of methods have been proposed.
The speed of sound, for example, at a certain temperature can be obtained by interpolation of values obtained from tables. The disadvantage of this approach is that often the values obtained are not sufficiently precise to enable a highly precise determination of a wall thickness as may ultimately be required.
Another method of determining the speed of sound at a certain temperature is to heat a tubular sample with a known wall thickness to the desired temperature and using a wall thickness measuring device which operates based upon the laser-ultrasound high wall thickness measuring technique, namely a pulse-echo method, measuring the wall thickness. From the known wall thickness the speed of sound in the material can be determined by detecting the time interval between the applied signal and the echo and calculate it back based upon the temperature.
A disadvantage of this technique, however, is that at high temperatures tubular samples which are used rapidly tend to scale and develop oxide films or coatings or falsify the measurement results. Furthermore, at the measuring points material must be removed because of the scaling to allow the laser-ultrasound hot wall thickness measurement technique to be employed so that the sample must be removed between two measurements.
It is, therefore, the principal object of the present invention to provide an improved method and apparatus with which these drawbacks can be obviated, namely, a method of and an apparatus which will allow the speed of sound in a material to be obtained without concern for scaling and in a highly precise and reproducible manner which especially enables thickness measurements to be calculated by the pulse-echo technique.
Another object of this invention is to provide a method of measuring the speed of sound in a workpiece which is simple and effective, does not require significant movement of the material in which the measurement is to be made, and can provide results of such precision that wall and other calculations can be made with high precision.
These objects are achieved, in accordance with the invention in a method of determining the speed of sound in a material as a function of the temperature by:
a) Initially preparing a sample body which is elongated, composed of the material in which the speed of sound is to be measured and which is formed preferably at one of its end regions with two reflection zones at a predetermined distance or spacing form one another.
b) Then at least the end region of the sample body provided with those reflection zones is heated to a temperature at which the speed of sound is to be determined.
c) An ultrasonic signal is then applied to the sample body.
d) The time interval is then measured between two ultrasonic echo signals emitted by the sample body and resulting from reflections of the ultrasonic signal applied to the sample body at the two reflection zones.
e) Finally the speed of sound is calculated as a quotient of the spacing between the reflection zones and the measured time interval.
More specifically the method of determining the speed of sound (c) in a material as a function of the temperature (T) can comprise the steps of:
(a) providing a sample body of the material at an end segment with two sound-reflection zones at a predetermined distance apart (a) in a longitudinal direction in the sample body;
(b) heating at least the end segment of the sample body to a temperature (T) at which a speed of sound (c) is to be determined;
(c) launching an ultrasonic signal into the sample body;
(d) measuring a time interval (xcex94t) between respective ultrasonic echo signals generated at the sound-reflection zones; and
(e) calculating the speed of sound (c) as the quotient of the distance (a) and the time interval (xcex94t) (c=a/xcex94t) for the temperature (T) to which the end segment is heated.
To determine the functional relationship between the speed of sound and the temperature, the steps (b) to (e) are repeated at different temperatures (T). When the sample is composed of metal, especially steel, the measurement of the speed of sound is preferably carried out at temperatures between room temperature and 1200xc2x0 C. in steps of 50 K. The measurement of the speed of sound at temperatures between room temperatures and 600xc2x0 C. can be made in steps of 100 K.
Advantageously, the two reflection zones are provided in an end region of the sample body which is uniformly heated and indeed thus can be the only part of the body which is uniformly heated. The opposite end, i.e. the end opposite the end which is heated, can be cooled and cooling can be provided for all of the nonheated parts of the sample body.
The ultrasonic signal which is launched into a sample body can be applied thereto at the end opposite the heated end and preferably from a piezoultrasonic element which can be coupled to the sample body by water coupling.
The apparatus for determining the speed of sound can comprise:
a heater for heating an end segment of an elongated sample body of the material provided with two sound-reflection zones at a predetermined distance apart (a) in a longitudinal direction in the sample body to a temperature (T) at which a speed of sound (c) is to be determined;
means for launching an ultrasonic signal into the sample body;
means for measuring a time interval (xcex94t) between respective ultrasonic echo signals generated at the sound-reflection zones; and
means for calculating the speed of sound (c) as the quotient of the distance (a) and the time interval (xcex94t) (c=a/xcex94t) for the temperature (T) to which the end segment is heated.
The sample body is preferably a round rod or a flat bar composed of the material in which the speed of sound is to be determined. The reflection zones can be formed as notches matched in the sample body. Alternatively, they can be formed by providing a step in the sample body. In the latter case the sample body can simply have a cross section reduction which is of a special shape.
The heating means of the end provided with the reflection zones can be a furnace in which the end of the sample body is received or which surrounds this end. The apparatus can also include means for cooling at least the end of the sample body opposite the heated end.
The means for launching the ultrasonic signal into the sample body can be any commercially available piezo test head as may be used for ultrasonic applications although other techniques for generating the ultrasonic signals may be used as well. For example, the transducer for producing the ultrasound may be an EMUS (electromagnetic ultrasound) generator.
It has been found to be advantageous to produce a sample body of a length of 750 to 1250 mm and the two reflection zones in the form of notches in one end of the sample body at a spacing of 50 to 200 mm from one another, preferably at a distance of 100 mm. The sample body can be a round rod whose diameter is between 15 mm and 50 mm and is preferably 30 mm.
With the system of the invention, the formation of scaling on the sample body does not have any effect so that a precise measurement of the sound speed can be obtained. Furthermore, material removal from the sample body by laser light plays no role in determining the sound speed.
The apparatus used can be very simple so that the variation in the sound speed with temperature can be obtained with precision and in spite of the low cost of the apparatus.